1. Field of the Invention
This invention relates to a thermal transfer recording medium. More particularly, the present invention relates to a thermal transfer recording medium for recording an image on a transfer medium (or image-receiving medium) such as paper, using a thermal head.
2. Description of the Related Art
Conventional methods of recording color images make use of a printing system such as offset printing and, in addition thereto, an ink-jet recording system, an electrostatic toner recording system or a thermal transfer recording system. In particular, the thermal transfer recording system can make compact an apparatus in which it is to be used and requires only simple maintenance. Hence, this system is widely used. In particular, a system making use of a sublimation dye (hereinafter "sublimation transfer system") can provide an image with an excellent gradation and is suitable for instances in which images are recorded in full colors.
In such a sublimation transfer system, recording mediums used are exemplified by those comprised of a lamination of a heat-resistant sliding layer, a substrate film and an ink layer comprising a sublimation dye and a binder resin, and transfer mediums used are those comprised of a substrate such as paper or plastic film and laminated thereto with a dyeable resin layer. Such transfer medium and thermal transfer recording medium are brought into pressure contact at the interface between a thermal head and a platen roll, and heat corresponding with image signals is applied to that interface from the thermal head, so that a transferred image is formed.
In conventional thermal transfer recording mediums, however, the sublimation dye contained in the ink layer causes a phenomenon of agglomeration and with time, gives a phenomenon of bleeding to the surface of the ink layer. This has tended to cause adhesion of the sublimation dye also to non-image areas of the transfer medium when transfer images are formed, and what is called background staining occurs, resulting in a serious lowering of image quality.
In order to prevent such phenomenons, it has been hitherto proposed to use as a binder in the ink layer a binder composition containing 90% by weight or more of polyvinyl butylal having a molecular weight of from 60,000 to 200,000 (Japanese Patent Application Laid-open No. 60-101087).
Such polyvinyl butyral, however, has so poor a fluidity that an ink making use of a binder resin containing it in an amount of 90% by weight or more lacks desired coating properties. In instances in which such an ink is applied to a substrate sheet to prepare a thermal transfer recording medium, coating uneveness may occur to cause what is called pinholes in the ink layer of the thermal transfer recording medium. Thus, there has been the problem that image qualities such as resolution are lowered when images are formed using a thermal transfer recording medium in which such pinholes are present.